1. Field of the Invention
The present invention relates to the technical field of phase-locked loop (PLL) and, more particularly, to a frequency synthesis system with self-calibrated loop stability and bandwidth.
2. Description of Related Art
In wireless receiving systems, a widely adaptive frequency synthesizer for television receivers, WiMax receivers and the like is a tough challenge on design. For each output frequency of the frequency synthesizer, the parameters (such as the frequency of an input signal, a multiplication factor, etc.) have to be accurately adjusted for minimizing the phase noises and maintaining the stability of the frequency synthesizer.
In the frequency synthesizer, the loop bandwidth indicates the response speed, and preferably is 1/20 of the reference frequency. The damping factor indicates the stability of the frequency synthesizer, and is preferably close to one. The systematic parameters above are related to special circuit parameters such as a charge pump current and the resistance of a filter. Therefore, the loop bandwidth and the damping factor can be varied with input/output frequencies, multiplication factors, and the like.
The various output frequencies and multiplication factors lead to different PLLs on design for different applications. Such a way increases complication in management and also development cost for different PLL designs and tests. A solution for this problem is to apply a complicated circuit and algorithm to the same PLL to meet with the various output frequencies and the multiplication factors. Namely, a single PLL is designed and applicable in different ICs. In such a design, the loop bandwidth and the damping factor require an automatic tuning mechanism to meet with various input frequencies, output frequencies and multiplication factors in different applications.
An adaptive PLL is able to adjust its parameters to meet with the desired output frequencies and multiplication factors. The adaptive PLL can reach a constant bandwidth-to-reference frequency ratio and a constant damping factor, regardless of processes, applied voltages and temperatures. Such features allow the bandwidth to be a fraction of a designed reference frequency, and the fraction is adjusted to reduce the phase noises of the voltage controlled oscillator (VCO), thereby optimizing the output phase noises of the adaptive PLL.
FIG. 1 is a block diagram of a typical PLL 100. In FIG. 1, the PLL 100 includes a phase detector 110, a charge pump 120, a filter 130, a VCO 140 and a frequency divider 150. When the PLL 100 is locked, the frequency of the signal CKOUT produced by the PLL 100 is N times of the frequency of the reference signal CKREF.
FIG. 2 is a block diagram of a typical PLL 100 in phase domain. As shown in FIG. 2, in phase domain, the phase detector 110, the charge pump 120 and the filter 130 can be equivalent to Kd×KF×F(S), and the VCO 140 is equivalent to
            K      VCO        s    ,where ΦO indicates a phase of the output signal CKOUT and ΦI indicates a phase of the reference signal CKREF. Accordingly, the transfer function
      Φ    O        Φ    I  can be expressed as:
                              ΔΦ          ->                                    Δ              ⁢                                                          ⁢                              I                CH                                      ->                                          Δ                ⁢                                                                  ⁢                                  V                  ϕ                                            ->                                                Δ                  ⁢                                                                          ⁢                  f                                ->                ΔΦ                                                    ⁢                                  ⁢                                                                              Φ                  O                                =                                ⁢                                                                                                    (                                                                              Φ                            I                                                    -                                                                                    Φ                              O                                                        N                                                                          )                                            ︸                                        ΔΦ                                    ×                                      I                    CH                                    ×                                                                                    (                                                  R                          +                                                      1                            sC                                                                          )                                            ︸                                                              F                      ⁡                                              (                        S                        )                                                                              ×                                      K                    VCO                                    ×                                                                                                                        ⁢                                                      1                    s                                    ⇒                                                            Φ                      O                                        ⁡                                          (                                              1                        +                                                                                                            I                              CH                                                        N                                                    ×                                                                                    F                              ⁡                                                              (                                S                                )                                                                                      s                                                    ×                                                      K                            VCO                                                                                              )                                                                                                                                              =                                ⁢                                                                            Φ                      I                                        ×                                          I                      CH                                        ×                                                                  F                        ⁡                                                  (                          S                          )                                                                    s                                        ×                                          K                      VCO                                                        ⇒                                                            Φ                      O                                                              Φ                      I                                                                                                                                              =                                ⁢                                                                            I                      CH                                        ×                                                                  F                        ⁡                                                  (                          S                          )                                                                    s                                        ×                                          K                      VCO                                                                            1                    +                                                                                            I                          CH                                                N                                            ×                                                                        F                          ⁡                                                      (                            S                            )                                                                          s                                            ×                                              K                        VCO                                                                                                                                                                    =                                ⁢                                                                            I                      CH                                        ×                                                                  1                        +                        sRC                                                                                              s                          2                                                ×                        C                                                              ×                                          K                      VCO                                                                            1                    +                                                                                            I                          CH                                                N                                            ×                                                                        1                          +                          sRC                                                                                                      s                            2                                                    ×                          C                                                                    ×                                              K                        VCO                                                                                                                                                                    =                                ⁢                                                                            I                      CH                                        ×                                          (                                              1                        +                        sRC                                            )                                        ×                                          K                      VCO                                                                                                                          s                        2                                            ⁢                      C                                        +                                                                                            I                          CH                                                N                                            ×                                              K                        VCO                                                              +                                          s                      ×                                                                        I                          CH                                                N                                            ×                      RC                      ×                                              K                        VCO                                                                                                                                                                    =                                ⁢                                                                                                    I                        CH                                            ×                                              K                        VCO                                                              +                                                                  I                        CH                                            ×                      sRC                      ×                                              K                        VCO                                                                                                                                                s                        2                                            ⁢                      C                                        +                                                                                            I                          CH                                                N                                            ×                                              K                        VCO                                                              +                                                                                            I                          CH                                                N                                            ×                      sRC                      ×                                              K                        VCO                                                                                                                                                                    =                                ⁢                                  N                  ×                                                            1                      +                      sRC                                                              1                      +                      sRC                      +                                                                        s                          2                                                ×                                                  NC                          /                                                      (                                                                                          I                                CH                                                            ×                                                              K                                VCO                                                                                      )                                                                                                                                                                                                                                        =                                    ⁢                                      N                    ⁢                                                                                  ⁢                                                                  1                        +                                                  2                          ×                          ς                          ×                                                      (                                                          s                              /                                                              ω                                n                                                                                      )                                                                                                                      1                        +                                                  2                          ×                          ς                          ×                                                      (                                                          s                              /                                                              ω                                n                                                                                      )                                                                          +                                                                              (                                                          s                              /                                                              ω                                n                                                                                      )                                                    2                                                                                                                    ,                                                                        (        1        )            where ωn and ζ can be expressed as:
                                          ω            n                    =                                                                      I                  CH                                ×                                  K                  VCO                                                            N                ×                C                                                    ⁢                                  ⁢                              ς            =                                          1                2                            ⁢                                                                    1                    N                                    ×                                      I                    CH                                    ×                                      K                    VCO                                    ×                                      R                    2                                    ×                  C                                                              ,                                    (        2        )            for ωn indicates the loop bandwidth and ζ indicates the damping factor. In a typical application, ICH is proportional to N in order to reduce the affection from the variation of the value N. However, a PLL product typically requires a constant loop bandwidth
      ω    n        ω    REF  and a constant damping factor ζ. From equation (2), it is known that
      ω    n        ω    REF  is not a constant, and the loop bandwidth ωn and the damping factor ζ are a function of the circuit parameters of the PLL 100. However, the loop bandwidth
      ω    n        ω    REF  and the damping factor ζ shown in equation (2) should be constant for different applications.
In addition, the PLL needs to provide a frequency range which is large enough. For example, a required frequency ranges in 10 to 100 MHz for a video processor and in few hundreds KHz for an audio processor, and in this case the corresponding processor requires a frequency which ranges from 500 MHz to 1 GHz, so the output frequency range is increased up to 3000 to 10000 times (a tunable multiplier of the VCO output frequency, ex: 1000M/100 kHz=10000). Due to the various output frequencies, it requires different PLL designs for different applications. Such a way increases the managing trouble and the developing cost for designing and testing the different PLLs. Another solution is to design a single PLL applicable for different ICs, but how to design a PLL for a wideband operation is a challenge.
When the frequency requirement is met, another challenge is in the purity of the PLL output frequency, i.e., the output jitter, or known as the phase noise, in terms of characterized specification.
The PLL can produce a pure and stable clock, but noises can affect the clock stability. The degree affected by the noises can be decided by measuring the amount of PLL output jitter. The PLL output jitter typically includes cycle-to-cycle jitter, periodic jitter and long-term jitter.
After the system is operated for a long time, the long-term jitter may affect the system significantly. The long-term jitter often makes the system working point drift. The periodic jitter is caused by the imbalance or current leakage of a charge pump, which produces a static phase offset between the output signal and a reference signal. In the deep submicron technology, due to the shortened processes, the current leakage is increased exponentially. In addition, for a low voltage and wide operating range requirement, a typical PLL has a very high VCO regulation sensitivity, which makes the periodic jitter become worse. The long-term jitter is caused by a VCO phase error. Namely, a current output signal accumulates the long-term jitter at each transition due to the accumulation feature of the phase error and the leading variation of a previous output signal in time axis.
Accordingly, there still are problems existed in the conventional frequency synthesis systems, and thus it is desirable to provide an improved frequency synthesis system to mitigate and/or obviate the aforementioned problems.